This invention relates to a system for combining signals of an array of electromagnetic radiating elements or sonar transducers to form a beam of electromagnetic or sonic energy and, more particularly, to a system which forms beams in a plurality of directions relative to the array. In the ensuing description, the term transducers will be utilized since the preferred embodiment of the invention was implemented in a sonar system. However, it is understood that the description applies also to the forming of beams of electromagnetic energy and that the term transducer includes radiating element for an implementation of the invention in a radar system.
An array of transducers is often employed for receiving and transmitting sonic radiant energy. Receiving beams of the radiant energy are formed by combining signals of the transducers by preserving the relative phase shifts of signals induced by a wavefront of radiant energy propagating across the array. In the case of long arrays, long in the sense that the time for a wavefront of radiant energy to pass across the array is longer than the reciprocal of the rate at which data is to be extracted from the received radiant energy, the arithmetic operations involving the combining of the signals of the radiating elements or transducers to produce an output sample of data are only partially completed when the arithmetic operations are begun for the next output sample of data.
This situation is most readily seen in the case of a sonar transducer array utilized in forming a transmitting or a receiving beam of sonic energy. Considering, by way of example, a sonar receiving array which is sufficiently long such that several milliseconds elapse as a wavefront of radiant energy propagates across the array, and considering further that data is to be extracted from the array at a rate of several kilohertz, it is apparent that many output samples of data must be extracted during the time elapsed by the wavefront in propagating across the array. In an array configuration wherein transducers are arranged along a straight line, a plane wavefront arriving broadside is incident upon all the transducers simultaneously while a plane wavefront incident in the end-fire direction is incident sequentially upon each of the transducers. A wavefront incident obliquely upon the array propagates across the array in less time than the propagation time of the wavefront in the end-fire direction. Thus, it is seen that the time in which the transducer signals are combined to form a single sample of output data varies with the direction of the incident wavefront relative to the array.
A problem arises when the foregoing sonar array is utilized for gathering data from many directions, it being desirable to form beams in each of the many directions. The problem arises by virtue of the numerous arithmetic operations required for forming each output sample of data for each beam, a succession of the samples being provided for each direction in which a beam is to be formed. A problem arises in the timing of these arithmetic operations in view of the fact that the time elapsed during the computation of an output sample varies from a time interval shorter than the interval between successive output samples to a time which is longer than many of such output sample intervals. This variation occurs, as noted hereinabove, in accordance with the various directions of propagation of radiation relative to the array. With the large arrays utilized in highly directive sonar systems and the relatively high data rates often used by modern sonar systems, the necessary computations for forming beams in a multiplicity of directions and for extracting data therefrom may well require computers which are too large to be readily accommodated aboard a small ship, both in terms of the physical size of the computer equipment as well as in terms of the personnel required to service and operate the computer equipment.
In addition, there is a problem of synchronization of the sampling of the transducer signals with the aforementioned computations. The transducer signals are sampled at a sufficiently high input sampling rate, for example, ten samplings during the interval of time that a sound wave propagates a distance of one wavelength, to ensure that beams are formed with little or no grating lobes and grating nulls; thus, the sampling of the input transducer signals need be accomplished at a rate which is usually higher than the rate at which the output samples are produced for any one beam.